Diesel engines are the preferred means of producing torque for use in a wide range of applications ranging from transportation, such as heavy-duty trucks and trains, off-road agricultural and mining equipment, to the large scale production of on-site electrical power. Diesel engines operate at high internal temperatures that can result in at least some of the nitrogen present in the engine at the moment of combustion combining with oxygen to form nitrogen oxides (including NO and NO2). Nitrogen oxides readily combine with volatile organic compounds in the atmosphere to form smog and are regarded as a pollutant. Virtually every industrialized nation regulates the levels of nitrogen oxides that can be legally discharged into the atmosphere.
The emission of nitrogen oxides can be reduced, for example using selective catalytic reduction (SCR) which includes dosing a reductant (generically, diesel exhaust fluid (DEF)) into engine exhaust prior to its exposure to a catalyst. The DEF reduces nitrogen oxide emissions by converting the nitrogen oxides to byproducts that are less toxic. Typical exhaust systems can incorporate a control system for injecting the DEF from a tank or reservoir into the exhaust.